Correct! You have TWO drives which are logically linked into One 'Fused' drive. It is not a SSHD which is similar! It is a single drive (SATA 2.5" or 3.5") which is a standard HD but has a SSD mounted for a deep cache. You can't see the SSD or alter it. You could put a SSHD into your system (Seagate makes a nice one) replacing your current HD drive.

You are also correct the current drive config (2 TB Fusion Drive) has a small blade SSD.

Before you alter anything you need to break the Fusion drive back to discreet drives otherwise you could have issues repurposing either drive.

As for replacing the blade SSD it's a lot more work to get to. Maybe its best to leave it and just use it for your boot drive and just put in the SSHD to get the speed you want here (hosting some of your apps and all of your data files).

@danj I'm thinking out loud here. The specs say: This model is equipped by default with a 2 TB "Fusion" Drive, which combines a 128 GB SSD and a 2 TB hard drive. It has a Serial ATA (6 Gb/s) connector for a 2.5" hard drive and a PCIe connector (PCIe 2.0 x4 NVMexpress interface) for the SSD.

Given the size of the existing SSD blade, I do not see how changing out to a SSHD in place of the 2TB standard drive would help. I would think adding a 2.5" SSD drive in a 960GB version and fusing it with the blade would work much better. 960 GB OCZ SSD

The newer 'Thin Series' 27" models have a 3.5" HDD. It's the 21.5" models that have the 2.5" EveryMac is in error here. I had asked Brock if we could add the details to help people he beat me to it ;-} I'll fire off a note for the correction.

I wouldn't fuse the drives. Besides right now High Sierra still doesn't support APFS with Fusion Drives or RAID.

A dual drive setup with the OS, Apps & Cache on the blade makes sense, then use either an HD or SSHD for the deep storage. If you're doing massive video or music editing you need an external RAID'ed Thunderbolt drive set for the speed and depth.

I was just wondering if you had any feedback on this. I actually have a 2017 iMac, into which I just put a 2TB 950 EVO, replacing the 2TB HD that was previous there. Right now I have the 2TB SSD fused with the 128GB NVME blade, but the read/write speeds are hovering around 500… which is not quite what I was hoping for (I actually think the regular 2TB fusion drive crushes this, in benchmark tests I’ve seen on YouTube).

I’m curious why an NVME+SSD fusion drive would be SLOWER than the HDD fusion setup. Any thoughts?

Reading your exchange above, I’m thinking perhaps better now to use the 128GB for booting, and the SSD for the home directory. But I remain curious as to why the NVME isn’t kicking in at least a bit, in my benchmark tests.

I have a similar setup and was planning on making the same upgrade. Did you ever find out why the performance was degraded? I would have thought it would have really improved the Fusion throughput having two SSD devices (albeit different transfer rates) vs. a SSD and HD.

These are easy to get to. You use a tool that looks and feels exactly like a guitar pick (and is readily available at any music store) to separate the screen from the case. You'll need the right double-stick foam tape to put it back together. iFixIt and OWC have tutorials. OWC has the right tape and a tool that looks like a plastic pizza wheel — faster than the guitar pick. Now that 2T SSDs are under $300 and a 2T blade can be had for around $700, it's an upgrade that makes sense. Getting rid of that mechanical HHD lets your iMac run a lot cooler, too — the energy savings will pay for itself over time. If you can find an Apple system pull, that's best for the blade. 1T are easy to find; 2T don't come up all that often and are expensive—an m2 with an adapter is 2x that of an SATA SSD but not nearly as fast as the Apple NVMe PCIe quad blade as found in the 2017 iMac and certain MacBook Pros. APFS does not run on fusion drives—even when it's a blade fused to an SSD.